Conventional ferrous powder metal bodies, or parts, produced by simple pressing and sintering, are known to have rather inferior mechanical properties; e.g., impact and fatigue strength, because of the presence of pores in such bodies. Methods for overcoming these inferior properties are based upon achieving full or nearly full density. One method for obtaining nearly full density is to infiltrate the bodies with copper or a copper alloy, a process which has been in common practice since the 1940's.
Inspite of the ability to achieve substantially full density by infiltration with a suitable infiltrant, only a small improvement in dynamic properties has been achieved over uninfiltrated ferrous powder bodies.
The impact strength of ferrous powder metal parts is important for many applications, e.g., gears wherein a critical region is at the root of the gear teeth with weakness at that point leading to gear failure; and in hammers for use in hammer type mill wherein a critical area is the area between the head and the shank. Imperfection in this area can lead to failure.
A conventional method for determining impact strength of specimens in the Charpy impact test procedure is described in the Metal Powder Industries Federation (MPIF) Standard 40, 1974 Metal Powder Industries Federation P.O. Box 2054, Princeton, N.J. 08540. In this test, unnotched specimens are formed into a defined rectangular shape having specified dimensions and are placed in a pendulum-type impact machine with a capacity of at least 110 foot pounds (15.2 m-kg). The impact strength is the average of three tests reported to the nearest foot pound. Standard 40 is incorporated herein by reference thereto. For purposes of the present application, the term impact strength, where used, shall mean unless otherwise noted, the strength values obtained following the Charpy-type test procedure outlined in Standard 40.
Another mechanical property of interest in the preparation of many ferrous powder metal parts is the tensile strength. This property, and the test for determining it, are described in MPIF Standard 10, also incorporated herein by reference thereto. An aspect of the tensile strength of a powder metal part is the elongation of the part that occurs prior to failure. In the present application, the tensile strength and elongation shall be given (unless otherwise stated) in terms of kips per square inch (ksi or thousands of pounds per square inch) and percent elongation (E%), respectively, following the procedure of Standard 10.
Parts made according to state of the art powder metallurgy technology, i.e., pressed and sintered or infiltrated, have very low impact strengths--typically only 3 to 20 ft. lbs measured by the unnotched Charpy Test. Higher impact strength would enable these low cost methods to be used for higher performance parts that are now made by alternative technologies that are more expensive, i.e., powder metal forging, hot pressing, injection molding, etc.
Copper in iron is known to enable the iron to precipitation harden. Iron also can be hardened by adding carbon and heat treating. The use of carbon and heat treatment is least expensive and virtually the most common way the strengh and toughness of steel is improved.
Prior patent application Ser. No. 755,282 filed July 15, 1985, now U.S. Pat. No. 4,606,768 dated Aug. 19, 1986, assigned to assignee of the present application, describes how to improve significantly impact strength of copper infiltrated steel by assuring the absence of erosion and local porosity (defined statistically in terms of pore volume and maximum pore size). Unnotched Charpy impact strengths as high as 130 ft. lbs at an ultimate tensile strength of 103 ksi have been obtained. Combinations of high impact and high ultimate tensile strength are sought in many engineering applications. The disclosure of U.S. Pat. No. 4,606,768 is incorporated by reference herein.
State of the art copper infiltration of iron and steel parts uses long infiltration times to ensure the most complete infiltration possible and improve tensile strength. Typically, the times range from 30 minutes to 90 minutes, although shorter infiltration times have been reported. For these times there is partial alloying of the copper with the iron due to dissolving and reprecipitation of the iron because of liquid phase sintering as well as solid state diffusion of the copper into the iron. In the areas where copper and iron are both present, optimizing heat treatment for impact toughness is complicated by both carbon and copper hardening mechanisms operating at the same time.
Several investigators (see U.S. Pat. No. 4,606,768) have attempted to obtain higher combinations of impact strength and tensile strength. Some of these investigators (i.e., Kuroki et al in 1973, Impact Properties of Copper Infiltrated Sintered Iron; Journal Japan Society Powder Metallurgy, July, 1973, Vol. 20., pages 71-79) have employed short infiltration times but were unable to obtain the desirable and large improvements reported herein.
In application Ser. No. 935,854, supra, the applicants found that it was possible to obtain even better combinations of impact strength and tensile strength by controlling the microstructure of the infiltrated steel in such a way that the diffusion of copper into the steel matrix is kept within a certain range. Control of the cleanliness of the steel matrix affords an additional improvement. Through combination of the improvements of the U.S. Pat. No. 4,606,768 and those disclosed in Ser. No. 935,854 it was possible to obtain impact strengths (unnotched Charpy) of over 240 ft. pounds, and ultimate tensile strengths of over 100 ksi. Also, with the improvement of Ser. No. 935,854, it was found possible to obtain unnotched Charpy impact strengths of 50 ft. pounds at a tensile strength of over 100 ksi at a low overall density of about 7.55 g/cm.sup.3. At such low overall density, conventional processing typically gives an unnotched Chapy impact strength of less than 20 ft. lbs.
The invention of Ser. No. 935,854 also provides an infiltrated ferrous powder metal body infiltrated with copper or a copper alloy characterized as having after infiltration an overall density of at least 7.5 g/cm.sup.3 and a diffusion depth of copper into the steel matrix of less than about 4 micrometers as determined by chemical etching or less than about 8 micrometers as determined by electron dispersive X-ray analysis (EDXA).
An important aspect of the invention of Ser. No. 935,854 conducive to staying within the diffusion depth parameters stated above is employing as the powder metal an iron powder having a carbon content in the range of about 0.3 to about 1.4%, based on the weight of the copper-free iron skeleton. The percent carbon is the amount by weight added to the iron powder for preparing a so-called "green part". During sintering and infiltrating, a portion of this carbon is lost due to the formation of carbon oxides, the oxygen content of the iron powder being the source of the oxygen. Carbon may also be lost through the formation of hydrocarbons with any hydrogen used in the sintering atmosphere. Typical losses amount to about 0.1 to 0.2% based on the copper-free steel skeleton.
In U.S. Pat. No. 4,606,768 as well as Ser. No. 935,854, we have taught how to obtain excellent combinations of impact and tensile strengths by various infiltration processes that produce specific structures. Examples in Ser. No. 935,854 show that a "two-step" process (sintering followed by infiltrating) gave properties significantly better than the "single-step" (sintering and infiltrating performed simultaneously) process. For example, Examples 13 and 15 in Ser. No. 935,854 gave impact strengths of 64 and 90 ft. lbs. and tensile strengths 100 to 110 ksi whereas two-step processing gave impact strengths of 200 ft lbs. or more at similar tensile strengths.
In this application, for purposes of clarity, reference will henceforth be made to "single run" and "double run" to indicate whether one or two separate furnace operations are used. The term "step" will be reserved to indicate whether one or more distinct temperature plateaus are used in any individual furnace run.